The standard method for recovering gold from its ores involves leaching the finely ground ore with alkaline cyanide solution to dissolve the gold. Depending on the ore being cyanided, varying amounts of metals such as copper and iron are also dissolved in the cyanide solution. After the cyanidation step, various processing methods are available.
Of particular use in treating ores with a high clay content, or which generate large amounts of slimes during grinding, is the Carbon-In-Pulp (CIP) process. In this process, activated carbon is added to the cyanide-ore slurry after leaching is complete. The carbon-containing slurry is agitated for several hours, allowing adsorption of the dissolved gold by the activated carbon, which is then separated from the mixture.
The carbon-in-pulp procedure removes only the gold, leaving the other metals and the cyanide in solution in the liquid phase.
This gold-barren solution has to be returned to the milling circuit, or has to be disposed of. Solutions with a high metal content cannot be returned to the milling circuit because of "fouling", which will lead to high cyanide consumption and lower gold recovery (dissolution) and their disposal is limited by environmental regulations.
A variation of CIP is the Carbon-In-Leach (CIL) process, in which cyanide leaching and carbon adsorption are carried out simultaneously.
The gold-containing carbon is separated from the slurry and the gold is stripped from the carbon by conventional methods. The residual gold-barren slurry has traditionally been discarded into tailing ponds. The gold-barren slurry contains cyanide and also varying amounts of base metals such as copper and iron, present as cyano complexes; these substances find their way into the deposited tailings.
Widespread concerns about environmental contamination and the resultant increasingly stringent government regulations with respect to permissible levels of substances such as base metals and cyanide in dumped material make disposal of such tailings increasingly difficult.
If one attempts to reduce the metal loading of the tailings by causing release of metals such as Cu and Fe into the pond water which is decanted back into the milling circuit, these metals accumulate and are adsorbed by the activated carbon, finding their way eventually into the final gold product at greater than acceptable levels.
There is a need for convenient methods of removing and recovering base metals and cyanide from the gold-barren solutions.